Thermal collapse of sigle-walled aluminosilicate nanotubes: transformation mechanisms and morphology of the resulting lamellar phases

The thermally-induced structural transformations are studied of three imogolite-type nanotube (NT) materials: i) proper imogolite (IMO, (OH)3Al2O3SiOH) with outer surface covered by Al-OH-Al groups and inner one lined by silanols; ii) methyl-imogolite (Me-IMO, (OH)3Al2O3SiCH3), in which at the inner surface silanols have been replaced by methyl groups, while the outer surface is unchanged, and iii) the material Me-IMO-NH2, obtained by grafting the outer surface of Me-IMO with 3-aminopropylsilane (3-APS). TG-MS analysis on the parent IMO only shows loss of water (up to ca. 700 K), while XRD indicates the formation of a lamellar phase, because of the mutual reaction of inner silanols. With both Me-IMO and Me-IMO-NH2, Mass spectrometry and NMR analysis reveals the occurrence of a more complex collapsing mechanism, basically due to the reaction of outer Al-OH groups and inner Si-CH3, following the cleavage of the NTs structure, yielding methane and transient Al-O-CH2-Si species. All three materials show a limited decrease in the interlayer distance caused by collapse, as well as a substantial residual porosity. It is concluded that the layered structure can be conceived as consisting in an overall buckled structure, the strong strain within the silico-alumina layer of the single-walled nanotube providing the driving force against a complete flattening. As a minor feature, decomposition of perchlorate species to chloride anions with release of molecular oxygen is observed with IMO, species which are trapped during the synthesis at the narrow interpores cavities.